The present invention relates to an internal combustion engine of the Diesel type for combustion of gas which is compressed to a high pressure suitable for supply to the engine, which engine has an injection system with injectors that inject liquid fuel into the combustion chambers of the cylinders at a high pressure.
Dual-fuel two-stroke crosshead engines of this type are known in which liquid fuel is injected in the form of fuel oil, typically acting as an ignition aid for the injected gas. The gas in the known engines with high-pressure injection is natural gas which is gaseous at its injection into the combustion chamber. An engine of this type is described, for example, in the Applicant's brochure "Large Diesel Engines Using High Pressure Gas Injection Technology" from 1991 and in the technical article "Development of the World's First Large-Bore Gas-Injection Engine" by T. Fukuda, P. Sunn Pedersen et al, paper D51, CIMAC 1995 at Interlaken, CH. In these engines, the natural gas is supplied via a pipe system supplying a well-defined gas quality, normally methane gas. The high-pressure injection of the gaseous natural gas provides the advantage that the engine can use different compositions of the natural gas. Thus the gas may be of pure methane or, for example, of methane and ethane, if they were fractionated together.
Engines of the Diesel type with supply of gas-based fuel are further known in a number of different designs, all with the common feature that the gas is injected or supplied at a low pressure of, for example, about 1-5 bar into the intake air of the engine and thus replaces part of the fuel oil, which may have environmental advantages in the form of lower particulate emissions in the exhaust gas. For examples of such engines, reference can be made to EP-A 0049721, which mentions supply of LPG (propane/butane) to the intake air, EP-A 0102119, which mentions supply of LPG or methane, and EP-A 0133777, in which compressed natural gas or LPG is added to the intake air. In the cases where the gas is supplied to the engine as a liquid fuel, both evaporation and mixing of the gas with the intake air take place before introduction into the cylinder, while in cases of supply of gaseous gas only mixing takes place.
The proportion of the gas out of the aggregate volume of fuel must not become too large when the gas is mixed with the intake air in an engine of the Diesel type because otherwise auto-ignition of the gas may occur during the compression stroke. It has been described as important in the known art that gas ignition can only take place in a controlled manner by means of fuel oil injection. The injection of oil can be controlled in the usual manner with a suitably accurate timing for achieving the desired operating characteristics of the engine.
In the known engines, which, as mentioned above, may use high-pressure injection of gaseous gas directly into the combustion chamber or may use supply of gaseous or liquid gas to the intake air of the engine, it is a condition for gas operation that the gas is refined or in another manner has obtained a predetermined and stabilised composition with a predictable behaviour as fuel in the Diesel engine so that the actual engine can be adapted to the specific fuel in its structural design. If one of the known engines designed for the supply of gas at a specific ignitability is suddenly supplied with gas which is considerably more ignitable, auto-ignition may occur during the compression stroke with consequent heavy operational disturbances for the engine.
The partial gas operation of the known engines may result in a considerable environmental advantage in as much as the engine combusts less oil which at combustion forms environmentally harmful compounds, which do not occur to the same extent at combustion of gas.